Radio propagation in cellular systems will from time to time suffer from fading and co-channel interference problems. Fading is caused by multipaths, i.e. the transmitted signal arrives at the receiver via a number of paths, each of which will have different path lengths. In a point to point microwave application, the fade will occur for 1% of the time or less. The effects of the fades can be reduced by increasing the transmitted power, equalizing the faded received signal or applying frequency, space or receiver diversity.
Cellular applications have more problems than line of sight microwave because of the wide area of coverage and mobile traffic. Also, line of sight propagation can never be guaranteed in cellular applications.
In today's cellular application where a 100 to 200 feet antenna tower will cover an area from 1 up to 10 miles in radius, the propagation path loss from a point to another point in the same cell could vary by as much as 100 db or more. The transmit power has to be high enough to cover any possible loss. This not only wastes power, but in the worst case, the interference propagation path loss to other cells may not be as high as the path loss in the same cell, thus causing interference and affecting the performance of other cells. Thus, increasing the transmit power does not necessarily solve the problem.
When the application moves to a smaller cell or serving area, interference problems do not disappear simply because of a lower antenna height (20 to 50 feet or 8 to 12 feet for indoor environment). Statistically, the propagation path loss is equivalent to the large serving area, and the problems remain proportional. In addition, an indoor environment introduces further obstacles like walls and furniture.
The other common problem, as indicated above, in today's cellular systems is interference directed to co-channel cells. At present, each cell in a cellular system is covered by a single omnidirectional antenna located at the center of the cell or by a directional antenna for each sector in a sectorized cell. The signal transmitted from each antenna relies only on the terrain or propagation loss to limit the interference to the co-channel cells. However, with the emerging indoor Microcellular communication, a centralized omnidirectional antenna approach might not provide effective coverage and interference control due to the irregular layout of the building.